Eliminating vm->main_index introduced in ea2ec4c3ed7d.

Eliminating vm->main_index introduced in ea2ec4c3ed7d.

The vm->main_index was only needed for the disassembler in the
accumulative mode.  It was used to prevent from disassembling the code
from the previous iterations in the CLI.

The same result can be achieved without introducing the VM level field.